Na[GeF5]·2HF: the first quarternary phase in the H–Na–Ge–F system

trans-Pentafluorogermanate dihydrogen fluoride represents the first quarternary phase in the system H–Na–Ge–F, and the third Na–Ge–F phase after the X-ray structure analyses of Na2GeF6 and CsNaGeF6. The crystal structure of trans-pentafluorogermanate dihydrogen fluoride consists of chains of [GeF6] octahedra and chains of pentagonal bipyramidal [NaF7] polyhedra. The Raman and IR spectra show the expected vibrational modes.


Introduction
Superacid chemistry can be applied as a powerful tool to isolate reactive volatile species by the formation of salts (Bayer et al., 2022;Leitz et al., 2018Leitz et al., , 2019)).These salts are mainly stabilized by F-atom interactions and are therefore more stable compared with the starting material.Furthermore, this offers the opportunity to estimate the acidity of compounds and structural parameters while widely retaining molecular corpus (Seelbinder et al., 2010).
Experiments and quantum chemical calculations revealed that the protonation of thiosulfuric acid is successful in the superacidic system HF/MF 5 (M = As, Sb) (Hopfinger et al., 2018).Investigations of the less acidic binary superacidic system HF/GeF 4 were performed to explore the structural chemistry of thiosulfuric acid and its protonated species.Since the H 0 value of the binary superacidic system HF/GeF 4 was assumed to be only slightly greater than for HF/AsF 5 -based systems, monoprotonation was expected.
It turned out that the reaction of sodium thiosulfate in HF/ GeF 4 led to the formation of Na[GeF 5 ]•2HF instead of protonation of thiosulfuric acid (see Scheme 1).Whereas no conversion of the sodium salts with the weakly coordinating anions [AsF 6 ] À and [SbF 6 ] À has been observed, the Lewis acid GeF 4 reacts with the formation of its sodium salt, i.e.Na- [GeF 5 ].
The obtained compound Na[GeF 5 ]•2HF is the first quaternary phase in the Na-Ge-H-F system.The crystal structure shows an unusual pentagonal bipyramidal coordination of Na by F, in analogy to IF 7 (Burbank, 1962;Christe et al., 1993).A similar coordination environment has not been observed for sodium yet, even for the related sodium hydrogen fluorides (Ivlev et al., 2017).The sodium hydrogen fluorides also consists of �-HF-linked polyhedra, such as the potassium hydrogen fluorides (Coyle et al., 1969(Coyle et al., , 1970)).
There is a rich structural diversity of [GeF 6 ] 2À -based anions which can be classified in analogy to silicates.The main differences are the octahedral coordination of germanium and connections of [GeF 6 ] 2À units via corners and edges.The most common anions are isolated, such as [GeF 6 ] 2À (neso), [Ge 2 F 10 ] 2À (soro) or [Ge 3 F 16 ] 4À .Octahedra chains of the anion can also be linked via cis or trans linkage, i.e. {[GeF 5 ] À } n , in analogy to inosilicates or can even form loop-branched chains, i.e. {[Ge 4 F 19 ] 3À } n (Soltner, 2011).Na[GeF 5 ]•2HF shows the rather rare structure element of trans-connected chains of pentafluorogermanates, similar to [XeF 5 ][GeF 5 ], the only representative so far documented by crystal structure analysis (Mallouk et al., 1984).

Experimental
Caution!Note that any contact with the described compounds should be avoided.Hydrolysis of GeF 4 and the synthesized salts forms HF which burns skin and causes irreparable damage.Safety precautions should be taken while handling these compounds.All reactions were carried out by employing standard Schlenk techniques on a stainless steel vacuum line.The syntheses of the salts were performed using FEP/PFA (fluoroethylenepropylene/perfluoralkoxy) reactors with stainless steel valves.

Synthesis and crystallization
Anhydrous hydrogen fluoride (80.04 mg, 4.0 mmol) and germanium tetrafluoride (297.16mg, 2.0 mmol) were condensed into an FEP reactor.The solution was warmed to 233 K and thoroughly mixed for 5 min.Sodium thiosulfate (158.11mg, 1.0 mmol) was added to the superacid after freezing it at liquid nitrogen temperature, and the solution was warmed to 233 K again and thoroughly mixed for 5 min.The volatile components were removed over a period of 12 h in vacuo at 195 K.The product was obtained as colourless crystals in quantitative yield.

Crystal structure refinement
Basic crystallographic data and details of the data collection and structure refinement are summarized in Table 1.The positions of the H atoms in the structure were localized in the difference Fourier map and refined without any restrictions (Table 1).Symmetry checks by ADDSYM (Spek, 2001(Spek, , 2003;;Le Page, 1988) supported the space groups Pbca and Pca2 1 when regarding the heavy-atom arrangement; however, the noncentrosymmetric space group was only supported when taking the F and H atoms into account, as shown in Fig. 1.In contrast to the H and F atoms, the Na and Ge atoms contribute to hypersymmetry.The structure was refined as an inversion twin.
The calculated moiety formula was adjusted from 'F20 Ge4, 8(F H), 4(Na)' with Z = 2 to 'Na Ge F5, 2(F H)' with Z = 8, since the space group is orthorhombic and all atoms occupy the general position 4a.Due to symmetry, it can also be seen that the chains of octahedra are not isolated [Ge 2 F 10 ] 2À but instead {[GeF 5 ] À } n units.

Analysis
The product was further analysed by low-temperature vibrational spectroscopy in order to confirm the conformation of the fluorogermanate anion.IR spectroscopic investigations were carried out with a Bruker Vertex-80V FT-IR spectrometer using a cooled cell with a single-crystal CsBr plate on which small amounts of the sample were placed (Bayersdorfer et al., 1972).For the Raman measurements, a Bruker MultiRam FT-Raman spectrometer with Nd:YAG laser excitation (� = 1064 nm) was used.The measurement was performed after transferring the sample into a cooled (77 K) glass cell under a nitrogen atmosphere and subsequent evacuation of the glass cell.The low-temperature spectra are depicted in Fig. 2.
Single crystals of Na[GeF 5 ]•2HF suitable for single-crystal diffraction analysis were selected under a stereomicroscope in a cooled nitrogen stream.The single crystal was prepared on a stainless steel polyamide micromount (see Fig. 3) and data collection was performed at 117 K on a Xcalibur diffractometer system (Rigaku Oxford Diffraction).For details of the data collection and treatment, as well as of the structure solution and refinement, see the supporting information.
Decomposition of the product was already identified at 238 K by detecting the development of vapour pressure with temperature.

Vibrational spectroscopy
The Raman spectra show a broad line (712-600 cm À 1 ) appearing at 665 cm À 1 for the terminal Ge-F vibration of the [GeF 5 ] À anion (654 and 622 cm À 1 ) [the frequencies in parentheses are from Mallouk et al. (1984)].The Ge-F stretching vibrations of the 1 1 [GeF 5 ] À chain appear at between 536 and 524 cm À 1 (526 and 518 cm À 1 ).The bands at  to the square-plane angle deformation modes.These vibrations are similar to the values reported by Mallouk et al. (1984), but the data suffers from overlap in the fingerprint area.
The lines at 1342 [�(SO 3 )] and 1158 cm À 1 [�(SO 2 )] are due to the decomposition of the solvent (H 2 S 2 O 3 ) according to Scheme 2, as are the bands at 3269 [�(OH)], 1067 [�(SO)] and 916 cm À 1 [�(SF)].It can be assumed that the sulfur dioxide released by the decomposition of thiosulfuric acid reacts with excess hydrogen fluoride to form fluorosulfinic acid, as well as traces of polythionic acids, as reported in the literature (Hopfinger et al., 2018).
Since the structural chemistry of fluorogermanates has not been fully understood, other anions, as calculated by Soltner (2011), were compared with the observed data.Therefore, vibrations were also assigned to [GeF 5 ] À in accordance with the literature.The final assignments of vibrations for Na-[GeF 5 ]•2HF are listed in Table 2.

Crystal structure
In the 1 1 [GeF 5 ] À chains (Figs. 4 and 5), the trans-connected [GeF 6 ] 2À octahedra are tilted 28.94 � with respect to each other, and the octahedra are connected by atoms F1 and F6 (Fig. 6).The chains are arranged along the b axis and bent at Ge  3).
The sodium ions exhibit an unusual distorted pentagonal bipyramidal coordination.The coordination spheres of Na1 and Na2 are built up from atoms F2-F4 belonging to one trans-pentafluorogermanate anion and from F7, F9 and F10 from the second trans-pentafluorogermanate anion, and four F atoms (F11-F14) belonging to HF molecules (Fig. 7).The �-Fbridged Na1-and Na2-centred polyhedra are trans-edgelinked, forming an infinite tilted chain extended along the b axis.The distances between Na1 and Na2 are 3.906 (2) and 3.934 (2) A ˚, respectively, and the Na-F distances range from 2.271 (2) to 2.610 (3) A ˚. Therefore, Na[GeF 5 ]•2HF displays similar Na-F distances, but with higher deviations, compared to NaH 4 F 5 (Table 4).The different distances of the �-HF bridges leads to distortion of the pentatagonal bipyramid by the germanium chains.

Figure 6
The coordination environments of Ge1 and Ge2.Displacement ellipsoids are displayed at the 50% probability level.

Figure 5
The crystal structure of Na[GeF 5 ]•2HF, viewed along the b axis.Nacentred and Ge-centred polyhedra are shown in purple and grey, respectively.

Figure 7
The coordination environments of Na1 and Na2.Displacement ellipsoids are displayed at the 50% probability level.Two very strong hydrogen bonds are formed, namely, F12-H2� � �F5 [2.499 (3) A ˚] and F14-H4� � �F8 [2.483 (3) A ˚]. Two medium-strong hydrogen bonds form the connections The given distances are derived from F� � �F interatomic distances.In accordance with the criteria given by Jeffrey (1997), the assignment of weak/strong hydrogen bonds shows short and directed contacts for strong and longer and nondirectional contacts for weaker hydrogen bonds.

Conclusion
Thiosulfuric acid could not be protonated in the superacidic system HF/GeF 4 as intended.However, thiosulfuric acid proved to be a solvent for the crystallization of new A[Ge x F y ] z salts (A = alkali or alkaline-earth metals) due to the balanced acidity, volatility and extraordinary solubility of fluorinecontaining metal salts.By exploiting this method, new structures of alkali or alkaline-earth fluorogermanates might become accessible.
Expanding the gaps between the infinitive chains might result in new structures or might cause conformational changes in the fluorogermanate chains.Following this procedure, the structural chemistry of fluorogermantes could become more comprehensive.In analogy to silicates, ring formation might be observed in compounds with large lowcharged cations.
It may also be possible to synthesize Na[GeF 5 ] in a simplified reaction of sodium fluoride in HF/GeF 4 and it may be possible to improve the spectroscopic data, as decomposition of the solvent (H 2 S 2 O 3 ) could be avoided.Since the investigations were originally aimed at the protonation of thiosulfuric acid, no futher attempt was made to figure out whether the presence of thiosulfuric acid is necessary as a solvent or if the reaction could also just succeed in anhydrous hydrogen fluoride.As the solubility of sodium hydrogen fluorides increases drastically in anhydrous hydrogen fluoride with higher hydrogen fluoride content at low temperature, it can be expected that without additional solvent the reaction needs to be heated to homogenize the product.Otherwise a mixture of NaH 4 F 5 and NaF may be obtained reacting with the Lewis acid GeF 4 , leading to a mixture of different Na [GeF 5 ]•nHF.
Furthermore, the levelling effect of sodium salts could be shown for GeF 4 -based systems, in analogy to BF 3 decreasing Lewis acidity under the formation of sodium salts.

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes.
Refinement.Refinement of F 2 against ALL reflections.The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 .The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement.R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.
Refined as a 2-component inversion twin.

Figure 1
Figure 1 Overlay of the structural models in Pca2 1 and Pbca (light-grey unit cell, lighter atoms), viewed along (a) the a axis, (b) the b axis and (c) the c axis.Only H and F atoms show distinct differences in their respective positions, justifying the noncentrosymmetric model.

Figure 3 (
Figure 3 (a) Diffraction pattern and (b) the prepared single crystal on a polyamide loop of the micromount.

Table 1
Experimental details.

Table 3
Structural comparison of Ge-F bond lengths (A ˚).